Add beads and gastown to CI cache, consolidate CI workflow

- Add packages/beads and packages/gastown with shared definitions
- Expose custom-beads and custom-gastown in flake packages output
- Consolidate CI from matrix (8 parallel jobs) to single job with loop
- Saves ~12 minutes of redundant nix-setup time per run
- Uses ::group:: for collapsible log sections per package

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/gastown/default.nix

@@ -0,0 +1,38 @@
+# Gastown package - multi-agent workspace manager
+# Takes src as argument so it can be called from both overlay and flake packages
+{ lib
+, buildGoModule
+, src
+, version ? "unknown"
+}:
+
+buildGoModule {
+  pname = "gastown";
+  inherit version src;
+  vendorHash = "sha256-ripY9vrYgVW8bngAyMLh0LkU/Xx1UUaLgmAA7/EmWQU=";
+  subPackages = [ "cmd/gt" ];
+  doCheck = false;
+
+  # Must match ldflags from gastown Makefile - BuiltProperly=1 is required
+  # or gt will error with "This binary was built with 'go build' directly"
+  ldflags = [
+    "-X github.com/steveyegge/gastown/internal/cmd.Version=${version}"
+    "-X github.com/steveyegge/gastown/internal/cmd.Commit=${version}"
+    "-X github.com/steveyegge/gastown/internal/cmd.BuildTime=nix-build"
+    "-X github.com/steveyegge/gastown/internal/cmd.BuiltProperly=1"
+  ];
+
+  # Bug fixes not yet merged upstream
+  patches = [
+    ./gastown-fix-validate-recipient.patch
+    ./gastown-fix-agent-bead-address-title.patch
+    ./gastown-statusline-optimization.patch
+  ];
+
+  meta = with lib; {
+    description = "Gas Town - multi-agent workspace manager by Steve Yegge";
+    homepage = "https://github.com/steveyegge/gastown";
+    license = licenses.mit;
+    mainProgram = "gt";
+  };
+}